The simple fact of the matter is that the Viper has a massive engine, possibly the largest engine in the world when it comes to a production car. 8.2 litres of cubic displacement means that the Viper makes a great deal of it's power at very low revs and tends to run out of puff after 5500rpm. If you're trying to get the Dodge to rev all the way to the redline, then of course it's not going to respond. The displacement of the V10 doesn't allow it to develop power at such high revolutions.
Put simply, you'll notice that Honda's Integra which has a 2.0 litre engine can develop horsepower at extremely high revs, but the larger the engine becomes it will develop peak power with much less RPM. If you're thrashing your Viper for top-end speed at 6,500RPM, then you're missing the meaty part of it's power and you should tune the top-speed so that the car is maxing out at around 5,500RPM. I guarantee you it will be a much quicker car.